Both photocouplers / optocouplers and solid state relays (photo MOSFET or optical-coupled MOSFET (OCMOS FETs) transmit signals while remaining electrically isolated, but there are some important differences.
1. Structural Differences
The figures below show the principal internal structures of a photocoupler and an OCMOS FET.
As shown in the photocoupler on the left, when the light emitting diode (LED) lights up the phototransistor, the light generates a photocurrent that flows from the collector to the base of the phototransistor.
Accordingly, when the LED does not light up, the phototransistor is cutting off, and when the LED lights strongly, a large photocurrent flows from the collector to the base and the phototransistor is turned on steadily.
Unlike when the base-collector is simply short-circuited, even if the collector-emitter voltage is less than the base-emitter forward voltage of a transistor, the photocurrent still flows and the phototransistor is conductive.
On the other hand, as shown in the figure on the right above, the OCMOS FET incorporates photovoltaic cells, and when the LED lights up, the photovoltaic cells charge the gate capacitance to increase the gate-source voltage, turning on the MOSFETs in the case of a make-type contact.
For a break-type contact, the FETs are conductive with no gate-source voltage. However, when the LED is lit, the photovoltaic cells bias the gate-source voltage reversely, cutting off the FETs.
When the make-type OCMOS FET is turned off, the photovoltaic cells not only stop charging but the internal discharger switch is automatically closed, forcing the gates to discharge. As a result, the gate-source voltage immediately drops.
Two FETs in an OCMOS FET are serially connected in reverse together. Therefore, when the OCMOS FET is conductive, both of the FETs conduct bidirectionally. However, when the OCMOS FET is not conductive, only the FET which is forward direction with the applied voltage cut off, while a parasitic diode of the another FET conducts
2. Characteristic differences
Because of these structural differences above, photocouplers and OCMOS FETs have the characteristic differences as follows:
- Although photocouplers conduct only DC(direct current) in the output, OCMOS FETs can conduct both DC and AC (alternating current) in the FETs
- Generally, the operating speed of photocouplers is microseconds or more rapid, while that of OCMOS FETs is as slow as milliseconds.
- Although the output conduction characteristics of the photocoupler vary depending on the input current value, those of the OCMOS FET are unrelated to the input current value.
- Generally and theoretically the photocoupler becomes conductive corresponding to an input. However, there are two kinds of OCMOS FETs: one kind that conducts (a-contact: Make-type contact) and one kind that breaks (b-contact: Break-type contact), when input is applied.
Therefore, although high-speed operation like a photocoupler cannot be expected for OCMOS FETs, OCMOS FETs can switch AC and also a large current in the ampere range with a small input current (as small as a few milliampere).
3. Application Differences
In general the photocoupler is used only for the transmission of a DC signal. Its applications include:
1. Pulse transmission (in conventional digital circuits)
2. Analog DC signal transmission (for error feedback circuits in switching regulators, etc.)
On the other hand, because an OCMOS FET's operating speed is slower than that of a photocoupler, it is rarely used for signal transmission.
However, because of the MOSFET's bidirectional conduction and low on-resistance features, it is mainly used as an "electronic switch" that intermits AC signals.
Therefore OCMOS FETs are also called an SSR (Solid State Relay).